The magnetic recording industry is progressing towards an aerial storage density of 1 Tb/inch2. At recording densities of this magnitude, the bit size in the recording medium needs to be on the order of 25 nm to 50 nm. Magnetic recorded regions of this size are susceptible to the superparamagnetic limit wherein the magnetized regions (bits) are no longer thermally stable over reasonable time periods and recorded information can be lost. The size of a magnetized region that is affected by thermal instability is dictated by the following expression: KuV/kBT>0 where Ku is the magnetic crystalline anisotropy energy density of the material, V is the volume of the magnetized region, kB is the Boltzman constant, and T is the absolute temperature. As V decreases, Ku needs to increase accordingly in order to maintain magnetic stability at ordinary temperatures. Materials do exist that have values of Ku that support thermally stable bits at ordinary temperatures but their coercivities are too high to allow switching under the magnetic field generated by the 2.4 T writer poles on conventional read/write heads.
Heat assisted magnetic recording (HAMR) is a technique devised to overcome the difficulty in writing to materials with high Ku. In heat assisted magnetic recording, a beam of energy, typically visible, infrared or ultraviolet light, is directed to the surface of a magnetic recording medium in order to locally raise the temperature of a small region with nano dimensions to decrease the coercivity and allow switching of the magnetization of that region. Following switching, the heated region rapidly cools to room temperature where the high coercivity insures stability of the written bit.
Prior art methods of focusing optical energy into sub 50 nm size spots include the application of solid immersion lenses (SILs), solid immersion minors (SIMs) and other means of focusing coupled with a near field transducer at a focal point to concentrate and direct the energy to a small spot on a recording medium. Solid immersion lenses and solid immersion minors can be two dimensional planar waveguides with a predominantly parabolic shape such that an electromagnetic wave traveling in an axial direction through the waveguide is reflected off the edges of the waveguide due to an index mismatch between the waveguide and its immediate surroundings. The reflected waves concentrate at (or near) the focal point of the parabola. The diffraction limited spot size realized by SIMs and SILs with currently known transparent materials is greater than 80 nm, which is too large for heat assisted magnetic recording. To overcome this problem, optical near field transducers (NFTs) are employed to concentrate the energy at the recording medium. Optical NFTs are typically metal pins. If the electric field at the focal point is parallel to the axis of the pin, the field can couple with the pin and generate surface plasmons that travel along the surface of the pin and emerge as a small spot of concentrated energy much smaller than that of the focal point alone. When the optical NFT is proximate the air bearing surface of the recording head, a small spot with nano dimensions is heated on the recording medium.
Two dimensional SIMs and SILs are difficult to fabricate. There is a need for simpler methods of supplying energy to a near field transducer for heat assisted magnetic recording (HAMR).